Category Archives: Hardware

Taming the Comb; Spurious/Sideband Troubleshooting with the RTL-SDR (MAJOLSURF.NET)

Previously I wrote about the bring-up of the MRF49XA Shield.  Recall that instead of the standard FSK spectrum I was met with an asymmetric comb about the carrier.

the problem

Decreasing the span we see are able to measure the comb spacing, about 80kHz.

SDRsharp Analysis

When viewed in this way, we see more of a traditional harmonic series from the carrier.  I’ll assume for now that the series is not an odd function; the waveform is not square or compressed.  Typically symmetric sidebands that reside xdBc from the carrier are indicative of A.M. distortion.  I’ll also chalk up the asymmetry seen in at wider bandwidths is due to heterodyning.

The first place I like to look when experiencing undesired A.M. spurs is the power supply.


Setting the DMM to AC mode and turning on the frequency counter, I find a dominant 78kHz AC component on my 3V3 supply.


That’s close enough for me to be a smoking gun.  A bypass capacitor is placed near the MRF49XA supply…


And the major sidebands are gone.  We can still see spurs about 35dBc at 200kHz; for now I’ll assume they are Frac-N PLL spurs and move forward.

SDRsharp Analysis cap installation


DIY: HF/VHF Portable Mini Tuner de ON6MU

By Guy, de ON6MU

Schematic fig1

QRP mini tuner matcher

RE-AT2HF6/P Parts list

  • alu box of 70mm X 40mm X 33mm

  • 2 female PL 259 chassis

  • C1 = variable capacitor of at least 300pF or better 500 pF

  • S1 = 10 or more position rotary switch

  • L1 = 0,7mm insulated copper wire, 6 turns par connection closely together, 9mm outside diameter (8mm inside)
    taps every 6 turns and the last two sections (L1′ and L1”) 4 turns spaced at 1mm and 3turns spaced by 2 mm.
    The first two sections has a ferrite core inside.
    Could be that lower frequencies needs higher inductance, experiment with by adding a core in the last few sections (see fig2)

  • I added two bolts on the alu-box chassis to – if needed – connect the tuner to ground or for using a counterpoise.

  • R1: 1.5k carbon 1/4w (non-inductive resistor); optional to allow drain of possible static build-up on the antenna (or use a 10mH inductor)

  • R2: 2 x 470 carbon 1/2w parallel (non-inductive resistor); optional to have some little protection during switching when using a carier, as the switch could open the the connection for a fraction of a second during switching.

The coil

  • Wind 6 turns x – 2 connections on your switch (in this project a 10-position rotary switch is used, being 8 times 6 = 48 + 4 + 3 = 55 turns) over a 7mm screwdriver (or simular obkject hi) and make a tap every connection. Solder each tap.

  • Solder each tap to each connection of your switch and stretch L1′ (being 4 turns) at 1mm spacing and L1”(being only 3 turns) at 2 mm spacing. You can replace L1” by 3 turns of silver wire to allow better Q on higher frequencies (VHF).

  • Minituner insides…


  • Alex VE7DXW changed 5 of the lower 6 wdg air coils with 1 wdg, 2wdg, 4wdg, 4wdg, 6wdg ferrite core coils, which gives higher L values
    Thanks Alex!


  • manual operation

  • frequency range (depending on the coil min & max inductance):
    (Up to 150Mc if: L1” is silver(plated) wire, High Q switch, minimum capacitance of C is small enough and close connections are used in respect to 50 Ohms impedance)

  • 10 Watt +-

  • direct feed through

  • small and compact design ideal for low power QRP transceivers, like the Yaesu FT-817, or of course for receivers…

  • connection for counterpoise/ground

If you elect to use an antenna tuner, it is extremely important that you understand exactly how to use tuners and what they can and cannot do. A few watts of RF can easily become lost in an incorrectly adjusted antenna matching device. The whole idea of a QRP station is to keep things simple and economical, so I cannot overemphasize the priority of a clean, efficient connection of the amplifier output to a resonant antenna.


Homemade aluminum box